1. Field of the Invention
The present invention relates to an electronic-component mounting head, an electronic-component mounting apparatus, and an electronic-component mounting method and particularly relates to the art of reducing the contact load applied by an electronic component to a substrate when the component is mounted on the substrate.
2. Related Art
In many cases, electronic components are mounted on a substrate, such as a printed circuit board, by using a suction device which applies suction to each electronic component to hold the component and which is movable upward and downward. Thus, an electronic-component mounting apparatus includes (A) a lift member which is moved upward and downward by a lifting device and (B) an electronic-component mounting head which is supported by the lift member. The electronic-component mounting head includes (a) a head frame, (b) a suction device which is supported by the head frame via a bearing such that the suction device is movable upward and downward relative to the head frame and which applies suction to an electronic component to hold the component, and (c) a biasing device which biases the suction device downward, and the head frame is supported by the lift member.
For example, an electronic-component mounting head of an electronic-component mounting apparatus described in Japanese Patent Application laid open under Publication No. 3-203251 includes a head frame, a suction device which is supported by the head frame such that the suction device is movable upward and downward, and a compression coil spring provided between the suction device and the head frame. Thus, the suction device is biased downward by a biasing force equal to the sum of the elastic force of the spring and the weight of the suction device itself. The downward movement of the suction device is limited when the suction device abuts on a stopper provided on the head frame. A sliding bearing, not shown in the drawings, is provided between the suction device and the head frame.
When an electronic component is mounted on a printed circuit board as a substrate, a lift member is further moved downward by a small distance after the component contacts the circuit board. This movement is done for mounting the component on the substrate with reliability, even though the component, the substrate, etc. may have manufacturing errors. Since the compression coil spring is compressed and the relative movement of the head frame and the suction device is permitted, the downward movement of the head frame is permitted even after the component contacts the circuit board. This also applies to the event that an electronic component is held by the suction device.
However, the prior electronic-component mounting apparatus suffer from the problem that it is difficult to reduce the contact load applied by an electronic component to a substrate. When the head frame is further moved downward after the component contacts the substrate, the weight of the suction device and the biasing force of the compression coil spring act on the component. In addition, even if the sliding bearing may be provided between the suction device and the head frame, some sliding resistance is produced between the suction device and the head frame and a force based on this resistance is applied to the component. Thus, it is difficult to sufficiently reduce the contact load (strictly, this load additionally includes the self weight of the component) and accordingly only limited sorts of electronic components can be mounted.
For example, in the case of an electronic component of a sort which is provided by a wafer and a small number (e.g., four) of solder bumps which are formed directly on the wafer and each of which has very small dimensions (e.g., 0.1 mm diameter and 0.1 mm height), it is needed to reduce the contact load to not more than 4 g, providing that at most 1 g can be applied to each solder bump. However, generally, the weight of a suction device is more than 4 g and accordingly this specific sort of electronic component cannot be mounted on a printed circuit board.